The invention relates to electric discharge lamps of the high pressure metal vapor type and is especially applicable to such lamps having a metallic halide fill.
High pressure metal vapor arc discharge lamps generally comprise an elongated arc tube made of quartz or fused silica, the ends of which are sealed either by blow molding or by having pinched or pressed seals. The arc tube contains a quantity of mercury along with an inert starting gas such as argon and is provided with electrodes at opposite ends extending through the blow moldings or supported by the pinch or press seals.
Metallic halide lamps contain, in addition to the mercury and starting gas, one or more metal halides such as sodium, thallium, and indium iodides, or sodium and scandium iodides. In commercial metal halide lamps, the arc tube is generally enclosed within a vitreous outer envelope or jacket provided with a screw base at one end.
Arc tubes have been made utilizing a so-called full press seal, wherein the entire end segment of a piece of quartz or fused silica tubing is collapsed and sealed off. This is done by pinching the ends of the quartz tube while in a heat-softened condition between a pair of opposed jaws to press the quartz about a foliated inlead supporting an electrode on its inner end. The jaws contact and compress only the end portions of the quartz tube, thereby forming the press or pinch seal.
The immediately adjacent quartz which is viscous at the instant of pinching assumes a generally rounded shape in the transition zone between the cylindrical main body of the arc tube and the press seal which may be referred to as the end chamber.
Another method which is widely used in the lighting industry for shaping the end wells in quartz arc tubes, is commonly known as "blow molding".
In the blow molding process, a hemispherically shaped cavity is formed by the press feet when the seal is made and a slight excess pressure of inert gas is applied to the inside of the arc chamber. This gas pressure forces the plastic quartz to expand into the mold thus shaping the end of the arc tube.
The shape of the end chambers, that is of the space around and behind the electrodes, will vary with the type of quartz, the wall thickness, the heat concentration and the nitrogen pressure build-up during pressing.
That the inside surface of the arc tube should be smooth and free of angular crevices, corners, or pockets has previously been reported, see for example, U.S. Pat. No. 2,965,698. However, the realization of this need has not yet resulted in any one method suitable for totally achieving the stated requirement, see, for instance, U.S. Pat. No. 3,939,538 which recites advantages of the blow molding method over the press seal method.
While the blow molding method does provide some degree of control over the shape of the end well, it does not eliminate or prevent the formation of end pockets where the press seal meets the arc tube.
In any metal halide lamp the color temperature is controlled by the coldest spot temperature in the arc tube, which in turn determines the vapor pressures of the radiating species.
To have a lamp with reduced color temperature variations, what is really needed is an arc tube geometry in which the cold spot temperature does not vary, or at least is less sensitive to changes in the lamp's operating or burning position.
Modifications of arc tube inner geometry are thus constantly being explored, due in part, to the difficulty of control experienced during press sealing and blow molding.
The present invention represents yet another method for the modification of the internal geometry of arc tubes, in this case for the purpose of reducing color temperature variations in metal halide arc tubes.
A surprising benefit derived from this invention was the discovery that a GTE Sylvania M100 arc tube, modified in accordance with the present teachings, would operate with equal efficiency in both the vertical and horizontal positions. Unmodified M100 arc tubes suffered from both reduced lumen output and increased color temperature when changed from the normal vertical operating position to the horizontal.